Fri., 10/17/08 Pelvis/Injury Prevention, Paper #27, 11:33 am OTA-2008
Inlet and Outlet Radiographs Re-Defined
William M. Ricci, MD (a,b-AO, Synthes; a,b,c,e-Smith + Nephew; a,b,e-Wright Medical
Technology, Inc.; e-OrthoVita); Martin Tynan, MD (n);
Washington University School of Medicine, St. Louis, Missouri, USA
Introduction: Standard plain radiograph imaging of the pelvis includes inlet and outlet views that are most commonly defined as being 45° from the anteroposterior (AP). These views, ideally, profile the boney anatomy of the pelvis. However, given the unique anatomy of the pelvis and variability in pelvic tilt from individual to individual, orthogonal projections (45° each from the AP) are unlikely to result in standardized profiles. We hypothesized that inlet and outlet views optimized to be tangent to posterior boney landmarks (defined herein as “true” views) vary significantly from the standard 45° inlet and outlet views and that these “true” projections may not necessarily be orthogonal to each other. The purpose of the study was to quantify the trajectory of “true” inlet and outlet views relative to clinically important posterior boney landmarks using pelvic CT reconstructions. These data have the potential to redefine standard inlet and outlet radiographic imaging of the pelvis and to provide surgeons a means to obtain patient-specific intraoperative true inlet and outlet Carm projections that accurately profile the relevant boney anatomy of the posterior pelvis.
Methods: Sagittal reconstruction CT slices were used to measure the following angles relative to the standard AP plane: “true” inlet (defined as tangent to the lower two thirds of the S1 anterior body) and “true” outlet (tangent to the cephalad endplate of S1). Also measured were angles for the actual orientation of the S1 neuroforamen, lines tangent to the anterior body of S2, posterior body of S1, and posterior body of S2. Standard pelvic CT examinations (patient supine) performed on 14 patients (average age 39 years [range, 23-71 years]) were used for measurements. Patients with displaced sacral fractures, pelvic ring disruption, or osteoporosis were excluded.
Results: The average angle for the true inlet and outlet views were 23.5° (range, 7-34; standard deviation [SD] 7.8) and 47.7° (range, 34-64; SD 9.0), respectively. The true inlet angle varied significantly from a standard 45° inlet (P <0.001). The actual orientation of the S1 neuroforamen averaged 57.2° (range, 46-73; SD 8.4). The inlet view tangent to the anterior S2 body was 30.3° (range, 14-47; SD 9.0), posterior S1 body was 32.4° (range, 16-43; SD 7.5), and posterior S2 body was 37.3° (range, 22-54; SD 9.1).
Discussion: One of the goals for skeletal radiographic imaging is to provide standardized views relative to boney landmarks and orientations. Our data indicate that the unique pelvic anatomy calls for nonorthogonal imaging to profile the relevant posterior anatomy. We recommend a 24° caudad tilt for a true inlet that profiles the anterior body of S1 and a 48° cephalad tilt for a true outlet that profiles the S1 endplate. An outlet view that looks “down the pike” of the S1 neuroforamen requires, on average, an additional 10° of cephalad tilt (57° from AP). Our data also indicate that the angles required for true inlet and outlet pelvic radiographs vary substantially from patient to patient. When CT data are available, we recommend using the sagittal reconstructions to provide patient-specific angles to guide intraoperative C-arm imaging.
If noted, the author indicates something of value received. The codes are identified as a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options; e-consultant or employee; n-no conflicts disclosed, and *disclosure not available at time of printing.
• The FDA has not cleared this drug and/or medical device for the use described in this presentation (i.e., the drug or medical device is being discussed for an “off label” use). ◆FDA information not available at time of printing. Δ OTA Grant.